This invention relates generally to methods and apparatus for correction of distortion of signals in a nuclear magnetic resonance (NMR) system, and more particularly to methods and apparatus for correction of distortion in such NMR systems caused by switching of gradient magnetic fields when such switching results in oscillatory B0 magnetic fields.
In at least one known NMR imaging device, nuclear spins are subjected to magnetic fields and excited by a spin excitation signal from a radio frequency transmitter. The magnetic field is uniform and homogeneous. The frequency of the spin excitation signal is such that a resonant matching occurs to a natural Larmor precession frequency for those magnetic spins to be excited. The excited nuclear spins precess about a direction of the homogeneous magnetic field vector at an angle that depends upon the strength and duration of the spin excitation field. If the homogeneous magnetic field varies with time, the precession frequency will also vary.
In addition, at least one known NMR and imaging device utilizes gradient magnetic fields for volume selective spectroscopy or imaging. Gradient magnetic fields are applied to encode volume regions of a sample and thereby allow for position sensitive measurements of the nuclear magnetic resonance signal. However, when the gradient magnetic fields are switched on and off, inductive coupling produces current flow in conductive elements of the device. These currents produce undesirable time-dependent magnetic fields that adversely affect signal measurement. An undesirable eddy current field component, i.e., a uniform B0 component, is one result of necessary gradient field changes. Uncompensated B0 eddy currents can lead to image quality problems such as ghosting or to degraded MR (magnetic resonance) spectroscopy performance.
In known systems, only exponentially decaying gradient and B0 eddy current errors have been recognized. However, the introduction of shielded magnets with shortened axial extent has resulted in drastically higher static magnetic fields near the edges of the gradient coil. These magnetic fields, in turn, produce higher forces and significant oscillatory eddy currents. Oscillatory B0 eddy currents cause unwanted side lobes in MR spectroscopy and artifacts in MR imaging.
It would be desirable to provide apparatus and methods for correcting for oscillatory B0 eddy currents in the presence of changes in the gradient field.